I learnt that Canon has a live chat function. I asked them. This is the answer I got:
“In general when changing resolutions the camera uses the data from the sensor to interpolate the resolution, this is done by merging and binning pixels to record lower resolutions on the camera so it will still show the same aspect ratio at different resolutions”
If it is binning, with the sensor 5184×3456 (18MP) to get 1920×1280 (Full HD), there would be roughly 3×3 binning (effective pixel size 12.9um), for 1280×720 (HD) there would be roughly 4×4 binning (effective pixel size 17.2um), and to get 640×480 (SD) would be roughly 7×7 binning (effective pixel size 30um).
Maximum fps for video is 30fps for Full HD and SD, and 60fps for HD.
Therefore, using a 2x Powermate on his C11 with his 700D camera in 1280×720 mode with least compression, that gives him 0.63″ per [effective] pixel amd 60 fps. Sounds reasonable. Given the compression, he’s likely better off with a dedicated planetary camera, but interesting exercise to figure out what the DSLR does.
James